Prime Exposure Factors II
By Professor Stelmark
Primary Factors
The primary exposure technique factors the radiographer selects on the control
panel are milliamperage, time of exposure, and kilovoltage peak (kVp).
Depending on the type of control panel, milliamperage and exposure time may
be selected separately or combined as one factor, milliamperage/second
(mAs). Regardless, it is important to understand how changing each separately
or in combination affects the radiation reaching the IR and the radiographic
image.
• Milliamperage mA
• Exposure time
• Kilovoltage
Milliamperage
The Electric current is a flow of electric charge through a conductive medium.
In electric circuits this charge is often carried by moving electrons in a wire
The SI unit for measuring the rate of flow of electric charge is the ampere or
milliampere. (1/1,000)
In radiography mA controls the number of electrons in transit between cathode
and anode per unit of time (second). mA setting is associated with the specific
temperature of the filament and the number of electrons being liberated. mA is
the unit of tube current.
The higher the mA the more x-rays are produced
100 mA
400 mA
mA
5 -600 mA
A
3 -5 A
mA affects the quantity of x-rays produced, it also affects the intensity and the
total exposure or primary signal. mA is doubled – exposure is doubled –
intensity is doubled.
mA
Exposure Rate
Optical Density
mA
time
mAs
50
1 sec
50
100
1 sec
100
200
1 sec
200
Exposure Time
Duration of the tube current and radiographic exposure. How long the electrons
are flowing between the cathode and the anode. The exposure time ids directly
proportional to the number of x-rays generated.
Exposure time affect the quantity of x-rays produced
It does NOT affect the intensity
Exposure time
Exposure
Optical Density
mA
time
mAs
100
0.5 sec
50
100
1 sec
100
100
2 sec
200
The quantity of x-ray photons in an exposure cannot be determined by either
the mA or the exposure time alone. Although mA determines the rate of x-ray
production, it does not indicate the total quantity, because it does not indicate
how long the exposure lasts. Exposure time does not indicate the total quantity
either, because it does not measure the rate of x-ray production. To determine
the quantity of radiation involved in an exposure, both mA and time must be
considered.
mA x Time (sec) = mAs
220 mA x 0.077 (sec) = 16.94mAs
mAs
Exposure
Optical Density
As the mAs is increased, the quantity of radiation reaching the IR is
increased. As the mAs is decreased, the amount of radiation reaching the IR
is decreased.
Changes in mAs have a direct effect on density.
A. Original image.
B. Decreased in density when the mAs is decreased by half.
C. Increase in density when the mAs is doubled.
mAs is the primary controlling factor of
the x-ray beam quantity
mA
time
mAs
400
0.5 sec
200
300
1 sec
300
200
2 sec
400
mA
time
mAs
400
0.5 sec
200
200
1 sec
200
100
2 sec
200
mAs formula calculations
mAs
mA
sec
10 mAs
200 mA
10 mAs/ 200 mA = 0.05 sec.
sec
0.05sec x 1,000 = 50 ms
10 mAs
mA
40 msec
40 ms = 40/1,000 = 0.04 s. 10 mAs/0.04 s = 250 mA
mAs
200 mA
1sec
200 mA x 1 s = 200 mAs
Kilovoltage
When the kVp is increased at the control panel, a larger potential difference
occurs in the x-ray tube, giving more electrons the kinetic energy to produce
x-rays and increasing the kinetic energy overall. The result is more photons
(quantity) and higher energy photons (quality).
kV
30 – 150 kVp
The kVp affects the exposure to the IR because it alters the amount and
penetrating ability of the x-ray beam.
Altering the penetrating power of the x-ray beam affects its absorption and
transmission through the anatomic tissue being radiographed. Higher kVp
increases the penetrating power of the x-ray beam and results in less
absorption and more transmission in the anatomic tissues
mAs is the primary controlling factor of
the x-ray beam quality
kVp2
Exposure rate
Optical Density
Primary Exposure Factors and Image Density
mA increase
mA decrease
Analog System
Time increase
Time decrease
kVp increase
kVp decrease
Primary Exposure Factors and Image Brightness
mA increase
mA decrease
Time increase
Same
Digital System
Time decrease
kVp increase
kVp decrease